The present invention relates to a method and apparatus for producing a multiplex hologram.
Recently, it has been tried in various ways as an aid in medical diagnosis and surgery to produce in a three-dimensional multiplex holographic image a three-dimensional image reconstructed from tomographic images obtained by prevailing modern medical imaging techniques such as CT (computed tomography), MRI (magnetic resonance imaging) or the like.
The multiplex hologram is produced in two steps. Namely, original images are photographed in the first step, and the photographed original images are used to record a hologram in the second step. More particularly, an object 100 is placed on a turntable 101, and it is photographed continuously by a camera 102 while the turntable is being rotated, as shown in FIG. 1. Note that for reconstructing a three-dimensional image using data obtained with CT or MRI, such original images are produced by processing the data by a computer beforehand, displaying the original image on a CRT (cathode ray tube), and photographing the image with camera. Next, the original images are used to produce a hologram. As a means of recording the original image, a variety of multiplex hologram (referred to as "hologram" hereinafter) production apparatuses has been developed. One such hologram production apparatuses (referred to as "holographic apparatus" hereinafter) is disclosed in, for example, U.S. Pat. No. 4,364,627.
Such a holographic apparatus comprises a laser (not shown), an object optical system 103 and a reference optical system 104 as shown in FIG. 2. The object optical system 103 projects an original image 105 as an object beam onto a film 106. It is disposed on an optical path along which the laser beam travels, and it is further composed of first to fifth cylindrical lenses 107 to 111, and a spherical lens 112. The reference optical system 104 is so arranged as to project onto the surface of the film 106 a reference beam having the same wavelength as the object beam, make the reference beam interfere with the object beam similarly projected, and produce and record an interference image on the surface of the film 106. The reference optical system 104 consists of sixth and seventh cylindrical lenses 113 and 114 disposed on the optical path through which the reference beam passes, and shares the above-mentioned fifth cylindrical lens 111 with the object optical system 104.
In the object optical system of the holographic apparatus of the above-mentioned construction, the object beam is shaped into parallel light beams after passing through the spherical lens 112 which then incident perpendicularly (in direction X) upon the surface of the film 106, as shown in, for example, FIG. 3. Therefore, when a white light beam, for example, is projected to a hologram produced by the holographic apparatus of such construction at the time of reconstruction as shown in FIG. 4, color shading is caused since the white light beam is incident upon, and diffracted at, the surface of the film 106 and the diffracted light beams are incident upon the viewer's eyes through angles which are slightly different from one another in a vertical direction toward the surface of the film 106. More particularly, a light beam in a red light region (R), that is, having a long wavelength .lambda.1 is incident upon the viewer's eyes 114 from the upper end of the film 106 at which the light beam is diffracted through a large angle, a light beam in a blue region (B), that is, having a short wavelength .lambda.3 is incident upon the viewer's eyes 114 from the lower end of the film 106 where the light beam is diffracted through a small angle, and further a light beam in a green light region (G) of intermediate wavelength .lambda.2 is incident from the intermediate portion of the film 106. In other words, there will be reconstructed and viewed an image stained in colors slightly different from one another in the vertical direction toward the film 106. Hence, the factor of magnifications of the reconstructed image are different vertically according to the light regions, whereby the image is distorted. Therefore, when such a hologram in which a diseased spot is recorded is viewed for a medical diagnosis or examination, correct viewing cannot be done, possibly causing a wrong diagnosis.
Also, in the reference optical system of the holographic apparatus of the aforementioned construction, the horizontal (Y-directional) component of the reference beam is divergently incident upon the surface of the film 106 through the cylindrical lens 111 from the lower portion slanted with respect to the surface of the film 106, as shown in FIG. 5.
However, when the hologram is reconstructed, the white light source lies at the center of a film formed in a cylindrical form, and the reconstructed light of the white light source beam is incident perpendicularly (crossing the direction Y) upon the film. So the recorded reference beam and reconstructed light beam are different in angle of incidence from each other, causing a distortion. Therefore, when a hologram produced by such conventional holographic apparatus is viewed for medical diagnosis or examination, the reconstructed image can not possibly be interpreted correctly.